My dad had the blown-up muzzle of an M14 that had been fired with mud blocking the last inch or less of the bore. (Dad wasn't the dummy who blew it up. He was a small arms repairman in the Army, and claimed the souvenir when the rifle was brought in for repair.) The split extended a good six inches back from the muzzle. A boresighter is a considerably greater obstruction than a little bit of mud.

I'm going to bet that if you had high-speed video of the event, you'd see the barrel bulging and splitting from the middle towards the muzzle, not from the muzzle first and moving back.

This would mean that even though the barrel has opened up and started to release pressure, the bullet's momentum is still pushing it down the barrel, splitting the barrel like a wedge splitting a log.

(I have no expertise or experience here, this is just a SWAG.)

(Speaking of high-speed video, I looked again at the video you linked to a couple of days ago, showing (among other things) a bullet traveling lengthwise through a water bottle, hoping to get some insight. Unfortunately, water only sprays out the ends; there is no side-splitting.)